Method for adaptively setting transmission parameters for a random access channel transmission uplink procedure in a wireless communication system

ABSTRACT

The present invention provides a method to enhance the RACH message transmission of a wireless communication systems. Therefore the known RACH procedure for uplink transmission is extended by additional steps in order to allow the usage of adaptive transmission parameters for uplink transmissions, preferably the usage of adaptive modulation and coding (AMC). This is advantageous because studies in high-speed downlink packet access show the possibility to increase the data rate of a downlink shared channel. The method according to the invention introduces a new RACH message preamble and enables the base station to estimate suitable adaptive transmission parameters like an AMC setting according to the current transmission conditions which are used during the RACH message transmission.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is for entry into the U.S. national phase under §371for International Application No. PCT/EP01/07340 having an internationalfiling date of Jun. 27, 2001, and from which priority is claimed underall applicable sections of Title 35 of the United States Code including,but not limited to, Sections 120, 363 and 365(c).

TECHNICAL FIELD

The present invention relates to a method of transmitting user data froma mobile device to a wireless communication system using adaptivetransmission parameters in order to enhance the total data rate of theuplink transmission. The present invention also relates to a mobiledevice using the method of enhanced uplink transmission. Morespecifically the invention relates to a method for using adaptivemodulation and coding for a random access channel transmission uplinkprocedure in a code division multiple access device.

BACKGROUND OF THE INVENTION

The benefit of adapting the transmission parameters in a wireless systemto the changing channel conditions is well known. In fact, fast powercontrol is an example of a technique implemented to enable reliablecommunications while simultaneously improving system capacity. Theprocess of modifying the transmission parameters to compensate for thevariations in channel conditions is known as link adaptation. Animplicit link adaptation technique is hybrid automatic repeat request(H-ARQ). There are many schemes for implementing H-ARQ, standardized asH-ARQ, H-AQR type II H-ARQ type III and subtypes used for retransmittingerroneous data wherein re ion is adapted to the transmission conditionsdepending on the ARQ scheme used for retransmitting. Another techniquerelated to the category of link adaptation is explicit adaptivemodulation and coding (AMC) which requires explicit transmissioncondition m measurements or similar measurements to select the fittingtransmission parameters. Link adaptation is also achievable usingadaptive selection of antenna which is for example implemented asmultiple input multiple output antenna processing.

Adaptive modulation and coding (AMC) is part of the techniques behindthe concept of high-speed downlink packet access and is one of the mainfeatures introduced by code division multiple access communicationservices.

The main benefits of AMC are higher data rates which are available forusers in favorable positions which in turn increases the averagethroughput of the cell and reduces interference variations due to linkadaptation based on variations in the modulation and/or coding schemeinstead of variations in transmission of power. Up to now, AMC is onlyapplied in downlink shared channel (DSCH) to enhance transmission datarates.

Since large data amounts are not only transmitted from a base station toa mobile device designated as downlink transmission but also from amobile device to a base station designated as uplink transmission it isnecessary to apply adaptive transmission parameters to the mobiledevice. User data are transmitted from the mobile device to the basestation using random access channel (RACH) including fixed modulation.Up to now there is no transmission method or procedure known,respectively, to enhance data rate provided by the RACH for users infavorable positions.

The usage of adaptive transmission parameters requires the measurementof the current transmission conditions. Therefore, the measurement hasto be performed in a way which ensures obtaining transmission conditiondata reflecting the conditions during transmission enhanced by the usageof adaptive transmission parameters.

Transmission optimization methods are known in the field of mobilecommunications. For example WO 98/18280 discloses a mobilecommunications system which allows for significantly reducing a periodof time which is required for establishing a random accessmobile-originated call. During establishing a mobile station transmits arandom access frame that includes a preamble and a plurality of fields.An initial random access frame is transmitted in a first slot with aninitial power level and subsequent random access frames are transmittedin following slots with a power level being of the initial power levelincreased stepwise by a power level offset. The random access frames aretransmitted until the mobile station receives an acknowledgement fromthe base station. The acknowledgement schedules the mobile-initiatedcall.

Moreover, U.S. Pat. No. 6,181,686 discloses a method which relates tothe optimization of data transfer modes for transmissions in cellulartelecommunication systems. The optimization is based on a controlling ofthe data transfer rate by the means a downlink quality measurement(performed by the mobile station) and an uplink quality measurement(performed by the base station). The quality measurements are performedduring data transmission and in case the quality measurements over- orunderflow one or more pre-defined threshold value(s) a decision tochange the currently employed data transfer mode is provoked. In case ofa decision to change a handshake message procedure takes place toinstruct the new data transfer mode selected. A number time slots usedfor data transmission is adapted such that in conjunction with the newdata transfer mode having a different data transfer rate substantiallyconstant overall data transmission rate is obtained.

SUMMARY OF THE INVENTION

The object of the present invention is to enhance the total data rate inrandom access channel uplink transmissions by extending the procedure ofa random access channel user data uplink transmission.

The known procedure of a random access channel user data uplinktransmission is extended to employ transmission parameters for thetransmission of the random access channel message part. Therefore, theprocedure has to implement a method to determine the transmissionconditions just before the random access channel message part istransmitted.

According to this aspect of the present invention the random accesschannel procedure is performed as follows: the mobile device randomlyselects one of the RACH sub-channels from the group its access classallows it to use and the signature is also selected randomly from amongthe available signatures, the downlink power level is measured and theinitial RACH power level is set with proper margin due to open loopinaccuracy, a RACH preamble (e.g. a 1 ms RACH preamble) is sentincluding the selected signature, the mobile device decodes acquisitionindicator channel (AICH) to determine whether or not the base stationhas detected the preamble and in case no acquisition indicator (AI) isdetected on the AICH the mobile device increases the preambletransmission power by a power ramp step given by the base station (BS),e.g. a multiple of 1 dB, and the preamble is retransmitted in the nextavailable access slot.

The method of the present invention is characterized by the followingadditional steps. In case an AI transmission is detected on the AICHfrom the base station, the mobile device transmits a RACH messagepreamble including transmission conditions related data to enable theuse of adaptive transmission parameters for the RACH message part. TheRACH message preamble is decoded by the base station and allows the basestation to estimate the transmission conditions based on the messagepreamble. In combination with the current network load and the estimatedtransmission conditions, the base station is able to determine suitableadaptive transmission parameters allowed for the mobile device. Thisadaptive transmission parameter setting is transmitted as an indicationmessage to the mobile device via the AICH. The indication message isreceived by the mobile device and decoded. The mobile device is now ableto transmit the RACH message part using the transmitted adaptivetransmission parameter setting to increase the total transmission rate.

Preferably, the RACH message preamble includes the data necessary forthe transmission parameter setting. Conveniently, the RACH messagepreamble includes a number for indicating how many transmissions of theRACH preamble are transmitted to the base station before an AI isdetected on the AICH. Additionally, the RACH message preamble includes apower ramp step used for transmitting the RACH preambles before the AIis detected on the AICH.

Besides the above described transmission condition data, additionalinformation like antenna system information, speed information of themobile device and so on are possible. The conditions data which have tobe transmitted by mobile device to the base station to estimate thetransmission conditions and to determine according adaptive transmissionparameters result from the method or procedure of the estimation anddetermination process.

The way of coding the transmission related data is not limited. Therealization of the RACH message preamble can be done in several ways. Acertain realization is explained below, wherein the transmissioncondition related data are coded in a common vector and spread with aRACH preamble signature. This is one possible way. It is also possibleto transmit the data in another way. The way of coding the conditionrelated data has to meet the requirement to be decodable by the basestation.

More preferably, the RACH message preamble includes the number of RACHpreambles transmitted to the base station until an AI is detected on theAICH and the power ramp step used for transmitting the RACH preamblesbefore the AI is detected on the AICH. A possible way to co-code thesetransmission condition data is modifying the known RACH preamble. TheRACH preamble is a complex value sequence including a preamblesignature. The preamble signature is chosen to include the transmissioncondition data and consists of 256 repetitions of a length 16 signatureP_(s)(n), n=0 . . . 15, defined as followed:

-   -   C_(sig,s)(i)=P_(s)(i modulo 16), i=0, 1, . . . 4095

The signature P_(s)(n) is chosen from the set of 16 Hadamard codes oflength 16. The number of RACH preambles transmitted to the base stationand the power level used for transmission are coded in a value vector B,defined as follows:

-   -   B=(b_(m), . . . ,b₀), m ε N, >2

A certain number of sub-values b_(i) are reserved for the coding of thenumber of RACH preamble transmissions, the remaining sub-values arereserved accordingly for the coding of the power level of thetransmission. The value vector B is spread with the signatureC_(sig,s)(i). The resulting value shall be worded as new signatureC_(sig,s)(i). The base station knows C_(sig,s)(i) from the transmissionof the last preamble. Therefore, the value vector B can be decoded fromthe base station and the data regarding the number of RACH preambletransmissions of the mobile device and the power ramp step fortransmission of the RACH preambles are available for determining theadaptive transmission parameter setting. All other necessary data likecurrent network load are always available from and by the base station.

As an example, according to the maximal RACH preamble transmissions itis assumed to code a maximal number of 32 RACH preamble transmissionsneeding 5 bits to be reserved for coding and 3 bits to code the powerramp step. Overall 8 bits will be enough to code both values and thevalue vector B will be denoted as follows:${B = {\text{(}\underset{\underset{{number}\quad{of}\quad{transmissions}}{︸}}{b_{7},b_{6},b_{5},b_{4},b_{3}}}},{\underset{\underset{{power}\quad{level}}{︸}}{b_{2},b_{1},b_{0}}\text{)}}$

The representation (b₇, b₆, b₅, b₄, b₃)=(0,0,0,0,0) means that the RACHpreamble was transmitted once, the representation (b₇, b₆, b₅, b₄,b₃)=(0,0,0,0,1) means that the RACH preamble was transmitted twice andso on. Accordingly, the representation of the transmission power levelis performed in the same way. The 3 bits enable the representation of 8different power ramp steps. If the necessity appears to code a largernumber of transmissions or more different power ramp steps, the valuevector B is easily expandable to serve the necessity according to theexample above.

More preferably, the adaptive transmission parameters are parameters toenable the employment of adaptive modulation and coding (AMC) for RACHuplink transmissions.

According to another aspect of the present invention, a computer programfor carrying out the method for adaptively setting transmissionparameters for a random access channel transmission uplink procedure ina wireless communication system is provided, which comprises programcode means for performing all of the steps of the preceding methoddescription when said program is ran on a computer, a network device, amobile terminal, or an application specific integrated circuit.

According to yet another aspect of the invention, a computer programproduct is provided comprising program code means stored on a computerreadable medium for carrying out the method for adaptively settingtransmission parameters of the preceding description when said programproduct is run on a computer, a network device, a mobile terminal, or anapplication specific integrated circuit.

According to yet another additional aspect of the present invention, amobile electronic device is provided, which is adapted to perform amethod as described in the foregoing description. The electronic devicecan be a computer, or a network device such as mobile terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail by referringto the enclosed drawings in which:

FIG. 1 shows schematically the important steps according to a RACHprocedure in 3GPP (3^(rd) Generation Partner Project), and

FIG. 2 shows schematically the important steps according to the methodof the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates the important steps of a standard RACH procedure.Both the RACH and the AICH are shown to illustrate the timely sequenceof the transmissions. The procedure is explained in an exemplary wayaccording to the current UMTS standard performed as follows:

-   -   1. Available uplink access slots in a next access slot set are        derived for a set of available RACH sub-channels within the        given access slot class (ASC). One access slot is selected        randomly among the ones previously determined. In case of no        available access slot an uplink access slot is selected randomly        corresponding to the set of available RACH sub-channels within        the given ASC from the next access slot set.    -   2. A signature is selected randomly from the set of available        signatures within the given ASC.    -   3. A preamble retransmission counter is set to a preamble        retransmission maximum.    -   4. A commanded or desired preamble power is set to the initial        preamble power level.    -   5. In case of exceeding the maximum allowed value of the        commanded preamble power the preamble transmission power is set        to the maximum allowed power. Otherwise the preamble        transmission power is set to the commanded transmission power.        It is possible that the commanded transmission power is too low        in certain situations so that the preamble transmission power        has to be set above the commanded transmission power. The        preamble is transmitted a first time using the selected uplink        slot, signature and preamble transmission power.    -   6. In case no positive or negative AI corresponding to the        selected signature is detected on the AICH downlink access slot        corresponding to the selected uplink access slot:        -   6.1. a next available access slot in the set of available            RACH sub-channels within the given ASC is selected.        -   6.2. a new signature is selected randomly from the set of            available signatures within the given ASC.        -   6.3. The commanded preamble power is increased by a power            ramp step. If the commanded preamble power exceeds the            maximum allowed power by 6 dB the mobile device passes an            error message and exits the random access procedure.        -   6.4. The preamble retransmission counter is decreased by            one.        -   6.5 The steps beginning with step 5 are repeated if the            preamble transmission power is greater than zero. Otherwise            the mobile device passes an error message and exits the            random access procedure.    -   7. In case of detecting a negative AI corresponding to the        selected signature in the AICH downlink access slot to the        selected uplink access slot the mobile device passes an error        message and exits the random access procedure.    -   8. The random access message is transmitted three or four uplink        access slots after the uplink access slot of the last        transmitted preamble. The transmission power of the message is        chosen according to the transmission power under consideration        of further parameters of the transmitted preamble.    -   9. The mobile device passes a confirmation message and exits the        random access procedure.

FIG. 2 illustrates the steps according to one aspect of the method ofthe present invention. The procedure is similar or analagous to theprocedure described in FIG. 1 up to the point of receiving the AI by themobile device (see step 7). As a next step the new signature is derivedfrom the signature C_(sig,s)(i) included in the last transmitted RACHpreamble and the value vector B which includes the necessarytransmission related data used for determining the AMC setting.Preferably, the value vector B is spread with the RACH preamblesignature C_(sig,s)(i) in order to gain the new signature C_(sig,s)(i).The new signature is included in the RACH message preamble andtransmitted to the base station. The base station decodes the messagepreamble to extract the value vector B and determines in combinationwith base station internal data related to the channel conditions theAMC setting. This setting is transmitted in the form of an AMCindication via the AICH. The mobile device accepts the AMC settings andtransmits the RACH message part using the current AMC to improve thedata rate and reduce transmission errors.

The scheme is not limited to CDMA. It can be also used in other systems,like TDMA. In other systems, the value vector B can also be sent in away without spreading. The contents of value vector B will not belimited to the number of transmission, power ramp step, etc. It cancontain any transmission related information. Also here only adaptivemodulation and coding for message part is mentioned. However, otheradaptive transmission parameters for message part can also be used. Thescheme keeps the merits of AMC, which means users in favorable positionse.g. users close to the cell site are typically assigned higher ordermodulation with higher code rates, while users in unfavorable positionse.g. users close to the cell boundary, are assigned lower ordermodulation with lower code rates. Furthermore, the scheme is an adaptivefast access scheme, it can use system margin as much as possible and assoon as possible, thus increase system throughput.

1. A method for using adaptive transmission parameters for a randomaccess channel (RACH) transmission uplink, comprising the steps of:selecting randomly one of available random access channel (RACH)sub-channels, selecting randomly an available signature (C_(sig,s)(i)),setting an initial random access channel (RACH) power level, estimatedby a downlink power level, with a margin, transmitting a random accesschannel (RACH) preamble with said random access channel (RACH) powerlevel including said selected signature, decoding an acquisitionindicator (AI) on an acquisition indicator channel (AICH) to determinewhether or not a base station has detected said random access channel(RACH) preamble, in case no acquisition indicator (AI) on saidacquisition indicator channel (AICH) is detected: increasing said powerlevel and retransmitting said random access channel (RACH) preamble witha new random access channel (RACH) power level increased at a power rampstep, characterized by the additional step of: in case said acquisitionindicator (AI) on said acquisition indicator channel (AICH) is detected:transmitting a random access channel (RACH) message preamble includingtransmission conditions related data to enable a usage of adaptivetransmission parameter setting for the random access channel (RACH)message part transmission; wherein said transmission conditions relateddata includes a number of random access channel (RACH) preamblestransmitted to said base station until said acquisition indicator (AI)is detected on said acquisition indicator channel (AICH) and includessaid power ramp step used for transmitting said random access channel(RACH) preambles before said acquisition indicator (AI) is detected onsaid acquisition indicator channel (AICH), receiving an indicationmessage on said acquisition indicator channel (AICH), wherein saidindication message contains an adaptive transmission parameter setting;and using said adaptive transmission parameter setting for transmittingsaid message part of said random access channel (RACH) transmission. 2.A method according to claim 1, wherein said random access channel (RACH)message preamble includes channel condition related data.
 3. A methodaccording to claim 2, wherein said random access channel (RACH) messagepreamble consists of 256 repetitions of a length 16 signature P_(s)(n),n=0 . . . 15, defined as follows: C_(sig,s)(i)=P_(s)(i modulo 16), i=0,1, . . . , 4095 and a value vector B, defined as followed: B=(b_(m), . .. ,b₀), m ε N, m>2, is spread with the signature C_(sig,s)(i) to formsaid new signature C′_(sig,s)(i); wherein said signature P_(s)(n) ischosen from a set of 16 Hadamard codes of length 16 and said valuevector B codes said transmission conditions related data; wherein acertain number of sub-values b_(i) are reserved for said coding of saidnumber of random access channel (RACH) preamble transmissions and saidremaining sub-values are reserved for coding said power ramp step;wherein said new signature C′_(sig,s)(i) is used for said random accesschannel (RACH) message preamble including said transmission conditionsrelated data.
 4. A method according to claim 3, wherein said adaptivetransmission parameter setting includes an adaptive modulation andcoding setting.
 5. A computer program product comprising program codemeans stored on a computer readable medium for carrying out the methodof claim 4 when said program product is run on a computer, a networkdevice, a mobile device, or an application specific integrated circuit.6. A computer program product comprising program code means stored on acomputer readable medium for carrying out the method of claim 3 whensaid program product is run on a computer, a network device, a mobiledevice, or an application specific integrated circuit.
 7. A methodaccording to claim 2, wherein said adaptive transmission parametersetting includes an adaptive modulation and coding setting.
 8. Acomputer program product comprising program code means stored on acomputer readable medium for carrying out the method of claim 2 whensaid program product is run on a computer, a network device, a mobiledevice, or an application specific integrated circuit.
 9. A computerprogram for executing a method for using adaptive transmissionparameters for a random access channel (RACH) transmission uplink,comprising program code means for carrying out the steps of claim 1 whensaid program is run on a computer, a network device, a mobile device, oran application specific integrated circuit.
 10. A computer programproduct comprising program code means stored on a computer readablemedium for carrying out the method of claim 1 when said program productis run on a computer, a network device, a mobile device, or anapplication specific integrated circuit.
 11. An electronic device forwireless communication systems, comprising means adapted to perform amethod according to claim
 1. 12. A method according to claim 1, whereinsaid random access channel (RACH) message preamble consists of 256repetitions of a length 16 signature P_(s)(n), n=0 . . . 15, defined asfollows: C_(sig,s)(i)=P_(s)(i modulo 16), i=0, 1, . . . , 4095 and avalue vector B, defined as followed: B=(b_(m), . . . , b₀), m ε N, m>2,is spread with the signature C_(sig,s)(i) to form said new signatureC′_(sig,s)(i); wherein said signature P_(s)(n) is chosen from a set of16 Hadamard codes of length 16 and said value vector B codes saidtransmission conditions related data; wherein a certain number ofsub-values b_(i) are reserved for said coding of said number of randomaccess channel (RACH) preamble transmissions and said remainingsub-values are reserved for coding said power ramp step; wherein saidnew signature C′_(sig,s)(i) is used for said random access channel(RACH) message preamble including said transmission conditions relateddata.
 13. A method according to claim 1, wherein said adaptivetransmission parameter setting includes an adaptive modulation andcoding setting.